The invention relates to a fourth-order digital multiplex system, comprising a transmitter having a digital multiplexer and a receiver having a digital demultiplexer for TDM transmission of a plurality of digital signals at a nominal third-order bit rate of 44 736 kbit/s over a common digital path between the said multiplexer and the said demultiplexer.
At present there are two types of hierarchies of PCM multiplex systems for which the CCITT has given recommendations as regards the methods of multiplexing, the interfaces and the line transmission systems (see CCITT Yellow Book, Vol. III, Fascicle III.3, Recommendations of the series G.700 and G.900, Geneva, 1981).
A first type of hierarchy, which has found general acceptance in Europe, is based on a first-order PCM multiplex system with a nominal bit rate of 2048 kbit/s for the transmission of 30 telephone channels, the A-law being used for encoding. Within this first hierarchy, multiplexing takes place in factors of four and the recommended nominal bit rates amount to 8448 kbit/s for the second order, 34 368 kbit/s for the third order and 139 264 kbit/s for the fourth order.
Particularly in the United States, a second type of hierarchy has been generally accepted. This type is based on a first-order PCM multiplex system with a nominal bit rate of 1544 kbit/s for the transmission of 24 telephone channels, the .mu.-law being used for encoding. Within this second hierarchy, the recommended nominal bit rate amounts to 6312 kbit/s for the second order, while of the two recommended nominal bit rates for the third order the bit rate of 44 736 kbit/s is employed in the majority of cases. For the fourth order within this second hierarchy the CCITT has not yet given a recommendation, but the digital transmission network of Bell System in the United States now employs for the fourth order a nominal bit rate of 274 176 kbit/s for the transmission of six digital signals at a nominal third-order bit rate of 44 736 kbit/s.
The long-haul digital line transmission systems for this fourth-order bit rate of 274 176 kbit/s can be realized by extending the already existing long-haul line transmission network with completely novel systems for this purpose. As, however, a very substantial part of the expense for any long-haul line transmission system is formed by costs that are needed for whatever system is used, such as the costs for the cable itself, laying the cable, building and equipping terminal stations and subterranean repeater stations, the practical implementation of novel digital line transmission systems preferably utilizes the already existing long-haul analog line transmission systems. In the case at issue, the analog line transmission systems of the type L4 and L5 having a capacity of 3600 and 10800 telephone channels, respectively, on each 3/8-inch coaxial cable and having nominal repeater spacings of 3218 m (2 miles) and 1609 m (1 mile), respectively, may be considered for use. For the transmission of digital signals at a bit rate of 274 176 kbit/s over these 3/8 inch coaxial cables the consecutive regenerative repeaters are spaced up to 1737 m (5700 feet) apart. This means that the existing L5-system can be used when the analog repeaters are replaced by digital regenerative repeaters, but this solution has the disadvantage that the existing system capacity of 10 800 telephone channels is reduced to a capacity of not more than 4032 telephone channels, while moreover a L5-system is not available for all desired routes. These disadvantages are obviated when use is made of the L4-system, which is available at a much larger scale. However, in the latter system it is not sufficient merely to replace all the analog repeaters by digital regenerative repeaters but it is then also necessary to arrange an additional repeater station for a digital regenerative repeater between every two consecutive existing repeater stations, which entails high additional costs.